A diesel engine which uses clean-burning DME (dimethyl ether), instead of light oil, as fuel is now attracting attention as means for reducing air pollution by diesel engines. DME fuel is a liquefied gas fuel unlike light oil as a conventional fuel. That is, DME fuel has a boiling point lower than that of light oil and evaporates at room temperature whereas light oil exist as a liquid at atmospheric pressure and temperature. Thus, when DME fuel is used in a conventional diesel engine, the DME fuel evaporates when the supply pressure to an injection pump is low. Thus, to supply liquid DME fuel to an injection pump, the supply pressure to the injection pump must be higher than that required to deliver light oil to the injection pump.
Thus, when DME fuel is used in a conventional diesel engine, the amount of fuel leaking through a gap between a plunger barrel and a plunger of an injection pump for delivering the DME fuel to a fuel injection nozzle of the engine into a cam chamber of the injection pump is greater than the amount of fuel leaking when light oil fuel is used due to the high supply pressure to the injection pump. In addition, DME has a lower viscosity than that of light oil and thus easily leaks through the gap, which increases the amount of fuel leaking through the gap. There is a possibility that the DME fuel leaked through the gap between the plunger barrel and the plunger flows into the cam chamber of the injection pump and is evaporated therein and then the evaporated fuel enters the crank chamber of the engine and is ignited therein.
Also, there is a possibility that when DME fuel remaining in the injection system after stopping the engine leaks from a nozzle seat of the fuel injection nozzle into a cylinder of the engine and is evaporated therein and then the evaporated DME fuel fills the cylinder, abnormal combustion such as knocking occurs at the start of the engine and the engine cannot be normally started, resulting in significant vibration and noise.
As a prior art to solve the problems, JP-A-Hei 10-281030 discloses a DME fuel supply device in which the gap between the plunger barrel and the plunger is reduced to decrease the amount of DME fuel leaking into the cam chamber of the injection pump. The prior art, however, only decreases the amount of leaked DME fuel and does not solve the problem caused by the leaking DME fuel.
A DME fuel supply device is known in order to prevent leaked DME fuel from entering the lubricating oil in the crank chamber in which the cam chamber of the injection pump and the crank chamber of the engine are separated from each other and DME fuel leaked through the gap between the plunger barrel and the plunger into the cam chamber of the injection pump and DME fuel remaining in the injection system after stopping the engine are retrieved to a tank by an electric compressor or the like.
Since the cam chamber of the injection pump of the DME fuel supply device and the crank chamber of the engine are separated from each other and since DME fuel leaked through the gap between the plunger barrel and the plunger into the cam chamber of the injection pump is retrieved to the fuel tank by an electric compressor or the like, it is possible to prevent evaporated DME fuel from entering the crank chamber of the engine. Also, since the DME fuel remaining in the injection system after stopping the engine is retrieved to the tank by an electric compressor, it is possible to prevent abnormal combustion such as knocking at the start of the engine caused by the DME fuel remaining in the injection system after stopping the engine.
However, even when the DME fuel remaining in the injection system after stopping the engine is retrieved to the tank by an electric compressor, it is impossible to completely retrieve DEM fuel remaining in a fuel passage between the outlet of an injection pump element and the inlet of a fuel injection nozzle since a valve of the injection pump element is closed when the injection pump element is in a non-injection state. Thus, the DME fuel in the injection system cannot be retrieved completely and there still remains the possibility of abnormal combustion such as knocking at the start of the engine.
This invention has been made in view of the above circumstances and it is, therefore, an object of this invention to provide a DME fuel supply device for a diesel engine which can retrieve DME fuel remaining between the injection pump element and the fuel injection nozzle after stopping the engine.